Generally, in a TOFMS, a preset amount of kinetic energy is given to ions originating from sample components to make the ions fly over a prest length of space. The period of time required for the flight is measured for each ion, and the mass-to-charge ratio of each ion is determined from the time of flight of that ion. Therefore, if, when the ions are accelerated and caused to start flying, there exist variations among ions with regard to position or amount of initial energy, there arise variations among the time of flight of ions having the same mass-to-charge ratio, which causes a decrease in the mass resolution or mass accuracy. One commonly known solution to this problem is an orthogonal acceleration TOFMS (which may also be called a “vertical acceleration TOFMS” or “orthogonal extraction TOFMS”), in which ions are accelerated and sent into flight space in a direction orthogonal to an incident direction of an ion beam.
Meanwhile, in recent years, in order to perform identification and structural analysis of a substance with a large molecular weight or a substance having a complex chemical structure, MSn analysis (which may also be called “tandem analysis”) is being widely utilized in which ions having a specific mass-to-charge ratio are dissociated in one to a plurality of stages by a method such as collision-induced dissociation, and mass spectrometric analysis of the product ions generated thereby is performed. Well known mass spectrometers that can perform MSn analysis include: a triple quadrupole mass spectrometer in which a collision cell containing a quadrupole-type ion guide (or other multipole-type ion guide) for dissociating ions is sandwiched by quadrupole mass filters that are disposed at the front and rear of the collision cell; an ion trap mass spectrometer that uses an ion trap which has both a function of separating ions according to mass-to-charge ratios and a function of performing a dissociation of ions; and an ion trap time-of-flight mass spectrometer in which the aforementioned kind of ion trap and a TOFMS are combined.
Further, a quadrupole-time-of-flight mass spectrometer (hereunder, referred to as “Q-TOFMS” in accordance with customary usage) is also known in which a quadrupole mass filter is disposed at the front of a collision cell and an orthogonal acceleration TOFMS is disposed at the rear of the collision cell in order to make use of the favorable performance of the aforementioned orthogonal acceleration TOFMS.
FIG. 3A is a schematic configuration diagram of a collision cell and an orthogonal acceleration unit in a Q-TOFMS described in Patent Literature 1. FIG. 3B is a view illustrating a potential distribution on an axis (in this case, an ion-optical axis) C in FIG. 3A. FIG. 3C is a timing chart of a voltage applied to an exit-side gate electrode and an orthogonal acceleration voltage illustrated in FIG. 3A.
As illustrated in FIG. 3A, in the Q-TOFMS, a linear ion trap (or ion guide) 51 is provided inside a collision cell 50 for dissociating ions, precursor ions having a specific mass-to-charge ratio selected with a quadrupole mass filter (not shown) are dissociated inside the collision cell 50, and resultant product ions (and precursor ions that were not dissociated) are temporarily held by the linear ion trap 51. Then, by temporarily lowering the voltage applied to an exit-side gate electrode 52 provided on the side of the collision cell 50, the ions are released from the linear ion trap 51 at a predetermined timing. The released ions are introduced along an X-axis direction into an orthogonal acceleration unit 55 of an orthogonal acceleration TOFMS via a grid electrode 53 and a skimmer 54, and when an acceleration voltage is applied to the orthogonal acceleration unit 55 at a predetermined timing, the ions are accelerated in a Z-axis direction and introduced into a flight space (not shown).
The solid line in FIG. 3B represents a potential distribution when ions are held in the linear ion trap 51. Since the potential of the exit-side gate electrode 52 is higher than that of the linear ion trap (rod electrode) 51, ions proceeding toward the exit-side gate electrode 52 are pushed back and contained inside the collision cell 50. The dashed line in FIG. 3B represents a potential distribution when the voltage applied to the exit-side gate electrode 52 is lowered. At this time, because the potential slopes downward from the exit-side end of the linear ion trap 51 toward the orthogonal acceleration unit 55, ions held up are accelerated toward the orthogonal acceleration unit 55.
Although ions having various mass-to-charge ratios that are held in the linear ion trap 51 are released almost simultaneously from the linear ion trap 51, there is a variation with respect to the ion travel direction (that is, the X-axis direction) until the ions reach the orthogonal acceleration unit 55. That is, because the acceleration energy imparted to each ion is substantially the same, the smaller the mass-to-charge ratio of an ion is, the higher the velocity of the ion is. Therefore, ions with a small mass-to-charge ratio travel ahead and arrive at the orthogonal acceleration unit 55 first, and ions with larger mass-to-charge ratios arrive at the orthogonal acceleration unit 55 with delays corresponding to the magnitude of the mass-to-charge ratios.
Because an acceleration voltage (a “push-pull voltage” in Patent Literature 1) is applied at a predetermined timing in the orthogonal acceleration unit 55, only ions that are passing through the orthogonal acceleration unit 55 during application of the acceleration voltage are accelerated toward the flight space, and the other ions are wasted. The utilization efficiency of the ions is called the “duty cycle”, and is defined by the following equation (see Patent Literature 2 and other related literatures).Duty Cycle [%]={(amount of ions utilized for measurement)/amount of ions that reach orthogonal acceleration unit)}×100
Ions having various mass-to-charge ratios are generated as a result of dissociating ions inside the collision cell 50, The Q-TOFMS described in Patent Literature 1 improves the duty cycle with respect to ions having a mass-to-charge ratio of interest, a delay time tD from the time point t1 of applying a pulse voltage for releasing ions from the linear ion trap 51 until the time point t2 of applying an acceleration voltage in the orthogonal acceleration unit 55 is adjusted in accordance with the mass-to-charge ratio of the target ions (see FIG. 3C). Since, by this means, an acceleration voltage is applied at a timing at which the ions of interest to the analyst pass through the orthogonal acceleration unit 55, the duty cycle for the ions of interest is improved and the detection sensitivity for the ions is enhanced. In this case, the duty cycle for ions other than the ions of interest to the analyst is low (or most of the ions are substantially not detected).
In a case where a mass-to-charge ratio of product ions to be observed is determined beforehand, MRM (multiple reaction ion monitoring) measurement or precursor ion scan measurement for example, the aforementioned Q-TOFMS is useful because the product ions can be detected with high sensitivity. However, when using the aforementioned Q-TOFMS, it is not possible to detect ions across an adequately wide mass-to-charge ratio range with high sensitivity, which is needed in the case of product ion scan measurement. That is, high duty cycle for ions cannot be achieved across a broad range of mass-to-charge ratios.
In addition to the aforementioned Q-TOFMS, a similar problem exists in the case of an ion trap time-of-flight mass spectrometer in which ions temporarily captured in a three-dimensional quadrupole ion trap are simultaneously ejected from the ion trap and subjected to mass spectrometry. In such a mass spectrometer, If ions arrive at an entrance of the ion trap in a wide spread group, among the ions that reach the ion trap, ions that may be captured inside the ion trap are the ions that arrive within a predetermined time range, and the other ions are reflected at the entrance or pass through the ion trap and are not utilized for measurement. Therefore, when ions arrive at entrance of the ion trap at a variety of arriving time depending on the mass-to-charge ratios of the ions, only ions within a limited mass-to-charge ratio range are captured, and ions across a wide mass-to-charge ratio range cannot be measured with high sensitivity.